The present invention relates to a corona device for use in modifying an electrostatic charge on dielectric surfaces, and more particularly to a corona erase device for use in an electrostatic printing apparatus to generate a supply of ions onto a rotating dielectric cylinder surface to modify or cancel any charge remaining on the cylinder surface after a latent electrostatic image has been transferred from the cylinder surface to a copy medium.
Corona devices are used both to place a uniform electrostatic charge on a dielectric surface and to eliminate an existing pattern of charge. Such actions are for the purposes of this description within the scope of the term "modifying an electrostatic charge on a dielectric surface".
The performance of a corona device is reduced by chemical compounds synthesized from the local air environment, which `grow` on the surface of the electrode. Dielectric toner can also accumulate on the surface of the electrode which produces localised charging and this reduces the magnitude and the consistency of the corona current. These effects can substantially shorten the useful life of the corona electrode thereby requiring a relatively frequent replacement of the entire corona assembly. It is therefore desirable to provide an assembly having more than one corona wire to minimise down time and simplify replacement of a useless wire.
One attempt to provide a multiple corona wire assembly is shown in U.S. Pat. No. 4,056,723 to Springett. This patent teaches a rotatable corona device for use with xerographic reproduction apparatus and having multiple electrodes mounted on a rotatable cylinder. Each electrode has a conductive biasing member associated with it to control the magnitude and polarity of charge deposited on the surface of the cylinder. The device is rotatable so that any one of the electrodes can be located at a desired operational position adjacent the surface onto which charge is to be deposited. If one of the electrodes should fail, or become inefficient, the device can be manually or automatically moved to the next position.
The assembly requires a considerable number of components and is quite complex resulting in relatively high manufacturing costs. In addition, should a conductive biasing member fail then the particular electrode associated with it can no longer be used even if it is still operating satisfactorily. Therefore although this device solves some of the problems associated with single corona electrode devices it has serious limitations due to its cost, and doubtful reliability and efficiency.
An improvement over the Springett structure is to be found in European patent application No. 84300633.9 to Delphax Systems and which was published in the European Bulletin of Sept. 19, 1984. This structure includes a plurality of axially aligned corona electrodes spaced angularly about an elongate support member. A conductive biasing member is wrapped around both the support member and the electrodes. The biasing member is preferably a wire wrapped helically and a conductor extends the length of the coil to ensure continuity should the biasing member fail locally. Structure is provided to permit the support member to be rotated to bring a selected one of the electrodes into position where it is energised for use.
The Delphax Systems structure is a marked improvement over that taught by Springett but also suffers from several disadvantages.
The replaceable member carrying the electrodes has to slide into a housing in the machine and it can be damaged as this is done. Further, while handling, the electrodes and wire can also be damaged or contaminated resulting in poor performance. Lastly, another disadvantage is that the user can rotate the member in either direction so that it is possible to turn the member back to an old position which brings a faulty electrode into position for use. Further it is not evident to the user whether the structure has any useful electrodes or possibly that a new structure is needed because all of the electrodes are faulty or inoperable.